Photo-imprinting techniques have been proposed as methods for forming micrometer and sub-micrometer size features on a substrate. In such techniques, patterns are formed by pressing an imprinting stamp or mold that has a preformed pattern on its surface, against a substrate having a layer (receiving layer) that can be imprinted. Both thermoplastic resins and photocurable resins can be used as the receiving layer. A thermoplastic resin can be heated above its softening point before imprinting and subsequently cooling to lower temperatures that cause the pattern to be fixed on the receiver layer surface before the imprinting stamp or mold is released. With photocurable resins, the imprinting stamp or mold is pressed against the receiver layer surface during irradiation (photo-imprint lithography). The resulting pattern can be fixed by photocuring. Depending on the nature of the photocurable resin, an additional thermal curing can be used before the imprinting stamp or mold is released. Such imprinting techniques are also known in the art as embossing or impressing.
A variety of known materials are useful for photo-imprint lithography. For example, photocurable compositions comprising a highly branched, multifunctional epoxy bisphenol A-novolac resins, such as Epon SU-8 from Momentive Specialty Chemicals Inc. have been described in the literature as high-aspect resists for thick-film applications. The photocurable compositions are generally formulated as solutions including an acid generating compound such as a di- or triaryl-substituted sulfonium or iodonium complex salt. The photocurable compositions can be applied to a substrate and dried to provide a dry coating thickness up to 100 μm. The dried coating can be photoimaged by exposure to UV light through a patterned photomask using contact, proximity, or projection exposures and then developed to form a high-resolution, negative-tone relief image of the photomask. Other performance benefits with the use of Epon SU-8 are its excellent thermal, chemical, and etching resistances when it is cured properly.
Recently, transparent electrodes including very fine patterns of conductive micro-wires have been proposed for various uses including touch screen displays. For example, capacitive touch screen displays having mesh electrodes including very fine line patterns of conductive elements, such as metal wires or conductive traces, are taught in U.S. Patent Application Publication 2010/0328248 (Mozdzyn) and U.S. Pat. No. 8,179,381 (Frey et al.). As disclosed in U.S. Pat. No. 8,179,381, fine conductor patterns are made by one of several processes, including laser-cured masking, inkjet printing, gravure printing, micro-replication, and micro-contact printing. The transparent micro-wire electrodes can include micro-wires that are 0.5 μm to 4 μm wide and exhibit a transparency in the display of 86% to 96%.
Fine patterns of conductive micro-wires can also be formed by inkjet printing conductive compositions (“inks”) onto a substrate followed by sintering the conductive compositions at a proper temperature, for example as described in U.S. Pat. No. 8,227,022 (Magdassi et al.) wherein it is disclosed to generate conductive patterns using aqueous-based silver nano-particle inks with multi-pass inkjet printing (5 passes or more) and sintering the printed patterns at temperatures of equal to greater than 150° C.
Moreover, U.S. Pat. No. 7,922,939 (Lewis et al.) discloses a silver nano-particle containing conductive composition having a silver concentration greater than 50% by weight. These conductive compositions can be considered as a high-viscous gel and have an elastic modulus value greater than the loss modulus value. However, the electrical conductivity generated by such conductive compositions is limited after annealing at high temperatures.
U.S. Pat. No. 7,931,941 (Mastropietro et al.) discloses a method of making a silver nano-particle dispersion using a carboxylic acid stabilizer resulting conductive films prepared at lower sintering temperatures. However, such dispersions cannot be readily formulated into conductive compositions.
WO2010/109465 (Magdassi et al.) discloses incorporating halide ions as a sintering agent into silver-containing dispersions or imprintable receivers to improve conductivity of the resulting patterns.
The art describes various forms of non-aqueous based silver nano-particle dispersions and some are commercially available. For environmental and safety reasons, it is highly desirable to have aqueous-based silver nano-particle dispersions. For performance reasons, it is highly desirable that these aqueous silver nano-particle dispersions are colloidally stable, can be prepared at high concentrations with low viscosities, are water reducible with excellent re-dissolution behaviors, and have excellent electrical conductivity after sintering.
Conductive micro-wires can be formed in micro-channels that have been embossed or imprinted into a photocurable composition on a substrate as described above. A photocurable composition can be applied to a suitable substrate. A pattern of micro-channels is embossed (impressed) onto the photocurable composition layer by a master (or mold) having a reverse pattern of ridges formed on its surface. The impressed photocurable composition is then cured by light before the master (mold) is released. An additional heat curing step can be used to further cure the composition. A conductive composition can be coated over the substrate, flowing into the formed micro-channels, and it is desired to remove excess conductive composition between micro-channels for example by mechanical buffing, patterned chemical electrolysis, or patterned chemical corrosion. The conductive composition remaining in the micro-channels can be cured, for example by heating.
The challenge for using such a method is to completely fill the micro-channels with the conductive composition without retaining residual conductive composition between the micro-channels. Yet, if the micro-channels are not completely filled with conductive composition, the conductivity of the micro-wires is significantly reduced and if the residual conductive composition is not removed, transparency of substrate and resulting conductive articles are impaired.
Besides the high transparency and conductivity that are desired, it is also desirable that the conductive micro-wires have good adhesion to the micro-channels on the substrate and are protected from scratches and other potential physical damage. Good micro-wire adhesion is required for flexible displays that can potentially experience a great deal of bending or flexing during device manufacture. Conversely, weak micro-wire adhesion can lead to the micro-wires popping out of the micro-channels and breaking.
Improvement in providing conductive patterns is described in copending and commonly assigned U.S. Ser. Nos. 14/017,638 and 14/017,707 (both noted above).
However, further improvements in this technology are needed whereby very narrow micro-wires of high aspect ratios can be formed to enhance the optical properties of the transparent conductive electrode without sacrificing its electrical performance, for example, the electrical conductivity. The overall challenges are how to form such narrow micro-channels of high aspect ratios and how to fill these micro-channels efficiently with conductive materials to form micro-wires that have low sheet resistance but without retaining residual composition between the micro-wires. With prior art technology and process, it becomes even more difficult to form such narrow micro-wires of high aspect ratios on flexible substrates that are more prone to heat distortion at relatively low to moderate temperatures, for example less than 150° C. If a high mechanical force is used to imprint micro-channels on such substrate, the heat distortion temperature will be even lower.
Thus, there is a need to develop technology for forming micro-channels on flexible substrates at relatively lower temperatures, for example less than 100° C. and at lower pressures. It is also desirable to develop a conductive material and a filling process to fill the micro-channels efficiently with the conductive material that will have a low sheet resistance value, and without retaining residual material between the micro-channels.